The dynamic behavior of the connecting rod and crank with elastic deformation is studied by using finite element method. By introducing Lagrange``s equation the coupled linear ordinary differential equations are derived from the system considered as the beam type structure. The resulting governing equations are solved numerically by using a modified Adams-Bashforth predictor-corrector method. The deformation of crank and connecting rod, and the resulting forces on the shaft have been investigated on the basis of the ratio of the crank to the connecting rod, the rotating speeds of crank, the ratio of the cross sectional area of the element, and the magnitude of gas force. In this paper it is found that the elastic deformation is influenced by both the angular velocity and the ratio of crank-connecting rod length, and the resulting forces and torque are mainly influenced by angular velocity. When the gas force is 100 lbf., the resultant forces of FMS1, FMS2, (the axial and transversal component of crank forces, respectively) and torque at 400 rad./sec.are about 100, 4000, and 400 times larger than those at 100 rad./sec. They are about 8, 400, and 50 times larger when the gas force is 1000 lbf. Thus the inertia force is found to be a major factor on force distributions in a high speed mechanism.